The problem that exists is to provide a way to use natural gas supplies that have a content of natural gas of from about 50 percent to 95 percent by volume hydrocarbons with the remainder primarily being nitrogen. A natural gas product, which consists primarily of methane, but which can contain small quantities of higher hydrocarbons and significant amounts of nitrogen cannot be sold as a natural gas fuel unless it contains at least 1000 Btu per standard cubic foot(scf) of natural gas. This is a standard in the industry. A consequence of this standard is that there are supplies of natural gas in the ground that cannot be used. These are wells that have been capped for lack of a market for this quality of gas. The natural gas content of these supplies range down to 50 percent by volume and lower. These supplies must be upgraded for use. The various other contaminating gases must be removed.
A classical way to remove contaminating gases from a natural gas stream is through liquefaction. In these processes the contaminating gases, which primarily is nitrogen, are separated from the hydrocarbon gases and vented to the atmosphere. If the natural gas contains quantities of sulfurous gases such as hydrogen sulfide, and water and carbon dioxide these can be removed in a prior step by scrubbing with monoethanolamine to remove hydrogen sulfide followed by drying with diethylene glycol, triethylene glycol, alumina, silica gel or zeolites. Optionally, a pressure swing adsorption technique such as disclosed in European Patent 394,947A can be used to remove carbon dioxide. In liquefaction processes the gas stream containing primarily methane but also amounts of higher hydrocarbons and nitrogen is cooled to recover the higher hydrocarbons as liquids and the stream then further cooled to liquify methane which is recovered in pipeline purity and used. The remaining gas, nitrogen, can be collected as a product or vented. As an option the methane/nitrogen stream can be processed to recover helium if helium is present in a sufficient amount.
It also is known to enrich natural gas using pressure swing adsorption techniques. In U.S. Pat. No. 5,171,333 there is disclosed a technique using four adsorbent beds, each of which contains a faujasite adsorbent. Each bed in sequence goes through an adsorption step, a desorption step by lowering the pressure and then a repressurization step to bring a bed back up to adsorption pressure. The adsorption step is conducted at about 100 to 500 psia and consists of passing a feed gas into an adsorbent bed. The desorption step consists of cocurrently depressurizing an adsorbent bed and passing the gas to a bed undergoing repressurization, further cocurrently depressurizing the adsorbent bed and passing the gas as a purge gas to a bed undergoing purging, countercurrently depressurizing and collecting a methane/ethane stream and countercurrently purging the adsorption zone with a purge gas from another adsorbent bed and recovering ethane. The repressurization step consists of repressurizing the adsorbent bed by cocurrently passing a depressurization gas into the adsorbent bed and further repressurizing the adsorbent bed by passing a portion of the adsorption effluent from another adsorbent bed to this adsorbent bed.
In U.S. Pat. No. 5,174,796 there is disclosed a pressure swing adsorption process for enriching a natural gas stream which contains nitrogen. A carbon adsorbent is used and the natural gas is preferentially adsorbed. The process steps consist of cocurrent adsorption, a first cocurrent depressurization and the use of a part of this gas to repressurize another adsorbent bed, cocurrently depressurizing the adsorbent bed to a yet lower pressure and withdrawing a fuel gas stream, countercurrently depressurizing the adsorbent bed and recovering a product gas, countercurrently purging the adsorbent bed and recovering additional product gas, then countercurrently repressurizing the adsorbent bed in two repressurization steps with nitrogen gas from another adsorbent bed. This repressurization brings the adsorbent bed up to about the feed gas pressure.
These are interesting processes but they are not highly efficient in the enrichment of natural gas. The various liquefaction processes have a high capital cost and are expensive to operate. The pressure swing adsorption processes to date have not optimized the recovery of the methane product gas. The objective in increasing efficiency is to desorb essentially all of the adsorbed gas in as high a purity as possible as quickly as possible, utilizing the value of any off gases, and then put the adsorbent bed back into production as quickly as possible. This is what is accomplished in the processes of the present invention. In addition, there should be a clean separation of the nitrogen and methane. That is, there should be essentially no methane in the nitrogen gas stream which usually will be vented since methane is the primary product.